There is magnetic resonance imaging (hereinafter, referred to as “MRI”) as a technique of imaging and displaying the internal tissue of an object, such as a human body, and an MRI apparatus using this technique is known. In this MRI apparatus, a static magnetic field is applied to the object from the outside, and a high frequency magnetic field pulse (hereinafter, referred to as an “RF pulse”) that is a high frequency electromagnetic wave is further applied. Then, the nuclear spins of atoms that form the tissue of the object cause the precession, a nuclear magnetic resonance signal (hereinafter, referred to as an “NMR signal”) generated when the nuclear spins return to a stable state is measured, and the shape or function of a measurement target part of the object, for example, shapes or functions of the head, chest, abdomen, limbs, and the like are imaged and displayed in a two-dimensional manner or in a three-dimensional manner using the NMR signal.
Images captured by the MRI apparatus are very effective for medical diagnosis. Accordingly, these images are widely used when diagnosing the state of illness or injury. An imaging target when the MRI apparatus captures a medical image is a human body as described above, and it is necessary to assume a state in which there is already a serious problem in health, such as injury or illness.
For this reason, it is necessary to pay close attention to safety. There is an international standard on safety “IEC 60601-2-33, 3rd edition” regarding electromagnetic waves used for imaging by the MRI apparatus. According to this international standard, the amount of absorption of RF pulses into the human body per unit time and unit mass is defined as a specific absorption rate (SAR), the upper limit of the SAR value is set, and imaging should be performed under the imaging conditions in which the upper limit is not exceeded. Restrictions on the amount of irradiation of electromagnetic waves have been demanded so that the electromagnetic waves exceeding the upper limit of the SAR value are not emitted to the human body, which is an imaging target, in imaging using the MRI apparatus. For safety, it is necessary to keep the restrictions based on the standard.
An example of a method of keeping the international regulations regarding the amount of irradiation of electromagnetic waves is disclosed in PTL 1. The method disclosed in PTL 1 is a method of using the MR signal for determining the position of the object on the bed. In order to determine the position of the object on the bed, an RF pulse is emitted to the object, an MR signal is detected from the entire body of the object, and the position of the object on the bed is determined based on the detection result and the output of RF pulses is determined so as not to exceed the upper limit of the SAR value based on the detection result.